This invention relates to a method for purifying and concentrating N-phosphonomethylglycine in an aqueous solution containing impurities using weakly basic ion exchange resin.
N-Phosphonomethylglycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. N-Phosphonomethylglycine and its salts are conveniently applied in an aqueous formulation as a postemergent phytotoxicant for the control of numerous plant species. N-Phosphonomethylglycine and its salts are characterized by a broad spectrum activity, i.e. the control growth of a wide variety of plants.
Numerous methods are known in the art for the preparation of N-phosphonomethylglycine. For example, U.S. Pat. No. 3,969,398 to Hershman discloses a process for the production of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid utilizing a molecular oxygen-containing gas as the oxidant in the presence of a catalyst consisting essentially of activated carbon. U.S. Pat. No. 3,954,848 to Franz discloses the oxidation of N-phosphonomethyliminodiacetic acid with hydrogen peroxide and an acid such as sulfuric acid. U.S. Pat. No. 4,670,190 to Kleiner discloses a process for the preparation of N-phosphonomethylglycine by reacting aminomethylphosphonic acid and glyoxylic acid in a molar ratio of about 1 to 2 in an aqueous medium or aqueous organic medium at temperatures between 30.degree. and 100.degree. C. These references are only illustrative, since there are many other methods known in the art for preparing N-phosphonomethylglycine.
Regardless of the process by which N-phosphonomethylglycine is prepared, all of these processes produce aqueous streams, including waste streams, that contain N-phosphonomethylglycine and various by-products and unreacted starting materials, for example, N-phosphonomethyliminodiacetic acid, N-formyl-N-phosphonomethylglycine, phosphoric acid, phosphorous acid, hexamethylenetetraamine, aminomethylphosphonic acid, iminodiacetic acid, formaldehyde, formic acid, and the like. Because of the valuable nature of the N-phosphonomethylglycine, it can be seen that there is a need among those skilled in the art to be able to recover the valuable N-phosphonomethylglycine from such aqueous streams by purifying the N-phosphonomethylglycine and recycling the other products to an earlier stage of the process for further conversion to the desired N-phosphonomethylglycine, or discharging such aqueous streams to waste treatment facilities and recovering the N-phosphonomethylglycine before it is so discharged.
Now, according to the present invention, there is provided a method of purifying and concentrating the N-phosphonomethylglycine in the presence of more acidic and basic compounds in the aqueous streams. In one embodiment of this invention, there is provided a method of concentrating and purifying the N-phosphonomethylglycine in the aqueous streams by first separating the components that are more acidic than N-phosphonomethylglycine, and thereafter separating the N-phosphonomethylglycine from those components that are less acidic than N-phosphonomethylglycine.